1. Field of the Invention
The present invention relates to a thin rolled steel plate having a so-called drum shape in cross section in which the thickness of the plate gradually increases from both edge portions toward a central portion as a result of hot rolling or hot rolling and cold rolling, to a method of producing the same and to an apparatus for producing same.
2. Description of the Prior Art
Conventional steel plates available in the market have roundness of a so-called crown shape in which the thickness of the plate at both edges is thinner by about 0.01 mm than that of the central portion. Therefore, the thickness of the conventional plates are nearly equal, exhibiting flatness to a sufficient degree. However, if the thickness of the steel plate having equal thickness is greatly reduced to minimize the weight of outer plates used, for instance, for automobiles, the panel stiffness of the plate is markedly decreased in the central portions, depriving the plates of practical value.